The design of rotor blades in a gas turbine engine is of vital importance in terms of efficiency with which the gas flow passing through the gas turbine engine interacts with the blades especially of the at least one turbine of the gas turbine arrangement.
Rotating gas turbine blades must fulfill a multitude of material- and design criteria which consider high mechanical and thermal stresses acting onto the rotating blades during operation. Due to enormous centrifugal forces acting onto rotating blades and an enormous thermal load that must withstand the blades, the main task in the design work of blades is to combine a high degree on stiffness which shall avoid blade vibrations during operation and the possibility of active cooling to enhance load capacity, by providing cooling channels inside the airfoil of a rotating blade. In view of the before requirements an optimized airfoil shape is always sought to improve turbine aerodynamic efficiency.
Rotating blades are arranged in rows which alternate in axial direction with rows of stationary vanes. Every pair of rows includes one row of stationary vanes and one row of rotating blades which follows in downstream direction directly forms a so-called stage. All stages of the turbine are numbered in sequence beginning with the first stage at the inlet opening of the turbine comprising the first row of stationary vanes followed by the first row of rotating blades.
Normal operation of a gas turbine shows that the stationary vanes, e.g. of the first stage, are excitation sources for vibrations acting onto the following rotating blades in downstream direction in disadvantage manner. It is therefore an object of turbine development to reduce such excitation sources and/or to enhance possibilities of decoupling mechanism to reduce and/or to avoid vibration transmission and excitation onto rotating blades arranged downstream of vanes in the first stage.
An obvious intervention would mean to change the excitation sources itself, but a change of the vanes in the first stage is considered to be expensive and would raise a lot of development work. Proposals to vary the radial length of the blades, i.e. the span of the airfoil which extends from the blade root to the blade tip, would have an impact onto the annulus of the flow path through the turbine which would lead to a major impact on a developments schedule which in view of that is not favorable. Another approach of reducing the tip mass of the rotating blade by reducing the axial chord length of the tip chord, which concerns a straight line connecting the leading edge and trailing edge of the airfoil in the region of the blade tip, resulted in aerodynamic penalty and furthermore a desired frequency shift of the resonant vibration behavior of the rotating blade was not achieved. Finally it was thought about to change the blade material in view of a possible change of Young's modulus, but this idea was dropped because of low cycle fatigue limitations associated with conventionally cast and directionally solidified materials.
All approaches of a desired influence on the vibration behavior of the rotating blades especially arranged within the first stage of a turbine and the turbine aerodynamic efficiency show the complexity of the problem. Major mass redistribution in designing an enhanced shape of the airfoil of a rotating blade is also considered to be especially difficult because rotating blades of the front stages are actively cooled components which are hollow bodies containing a multitude of cooling channel for cooling purpose. The thin metal walls of the rotating blades have to be cooled intensively to fulfill target life. Also the aspect of increasing the shank length of a rotating blade was considered to influence the vibration behavior of the rotating blade itself but was not deemed to be favorable due to the fact that this approach would result in rotor limits at the fire tree region in which cooling air supply via rotor bores is provided so that the rotor outline would also have to be adjusted.
The document U.S. Pat. No. 5,525,038 discloses a rotor blade for a gas turbine engine which is optimized to reduce tip leakage through a tip clearance. The rotor blade provides a significantly bowed surface formed at the tip region extending from the leading edge to the trailing edge of the suction side of the rotor blade. The profile cross-sections along the span of the airfoil of the rotor blade do not vary significantly, at least the axial chord length of the airfoil along the whole span of the rotor blade remains unchanged.
The axial chord length is defined as the length of the projection of the blade, as set in the turbine, onto a line parallel to the turbine axis. This can be seen in for example David Gordon Wilson's “The Design of High-Efficiency Turbomachinery and Gas Turbines”, pp 487-492, published by the MIT Press, Cambridge, Mass., 1984, 5th printing 1991. Particular reference is made to the second figure on page 487.